docs: def BinToTri(self, a, b):
  
          '''
          Turn an a-b coord to an x-y-z triangular coord .
          if z is negative, calc with its abs then return (a, -b).
          :param a,b: the numbers of the a-b coord
          :type a,b: float or double are both OK, just numbers
          :return:  the corresponding x-y-z triangular coord
          :rtype:   a tuple consist of x,y,z
          '''
  
          if (b >= 0):
              y = a - b / np.sqrt(3)
              z = b * 2 / np.sqrt(3)
              x = 100 - (a + b / np.sqrt(3))
              return (x, y, z)
          else:
              y = a + b / np.sqrt(3)
              z = b * 2 / np.sqrt(3)
              x = 100 - (a - b / np.sqrt(3))
              return (x, y, z)